Ferrous metal products, including steel, undergo corrosion in the presence of moisture, for example when exposed to air or water. It is, therefore, common practice to apply a protective coating to the ferrous metal product for protection against atmospheric corrosion. Various protective coatings may be used to prevent contact between the ferrous metal product and the atmosphere, and thus to prevent or reduce corrosion. In some instances, the protective coatings are sacrificial coatings i.e. coatings that are intended to preferentially react with moisture, and thereby protect the ferrous metal product.
A common coating on ferrous metal products is a galvanized coating, and such coatings are used on a wide variety of ferrous metal products, including steel products. Scrap galvanized coatings may contain a number of elements in addition to zinc, examples of which include aluminum, iron and silicon. Examples of methods of application include coating the ferrous metal product by passing the product through a bath of molten zinc and by electrodeposition of zinc. Passing the ferrous metal product through molten zinc tends to form an intermetallic compound at the interface and relatively pure zinc at the surface.
Part of the feedstock in the manufacture of steel is often scrap ferrous metal products. Such products are a low cost source of feedstock. However, if the scrap ferrous metal product has a galvanized coating, the high temperatures used in the manufacture of steel cause the zinc in the galvanized coating to be volatilized. The volatilized zinc is normally collected, especially in the form of a dust that is frequently referred to as electric arc furnace (EAC) dust. Electric arc furnace dust may also contain a number of elements in addition to zinc, examples of which include iron, lead, cadmium and chromium. Over time, there tends to an accumulation of zinc in the steel manufacturing process. Consequently, there is a tendency for the zinc content of the steel to increase. This causes difficulty in the manufacture of steel that meets product and quality control specifications, especially for the content of zinc as a trace metal, and ultimately to the manufacture of steel with unacceptable properties.
The galvanized coating on ferrous metal products may be removed (stripped) by treatment with caustic solutions e.g. solutions of potassium or sodium hydroxide. Such treatment results in the formation of potassium or sodium zincate, and an aqueous alkaline solution containing the zinc values. Although such stripping of the coating from galvanized ferrous metal products allows the stripped metal product to be used as feedstock in the manufacture of steel, the consequence is an aqueous alkaline solution of zinc values that must be treated or otherwise be disposed of in an environmentally friendly manner.